The limit value is 125 µg SO2/m3 as a daily average, not to be exceeded more than three days in a year and to be met by 2005. Over the years 1997-2009 the total population for which exposure estimates are made, increased from 56 to 100 million people due to an increasing number of monitoring stations reporting under the Exchange of Information Decision. Year-to-year variations in exposure classes are partly caused by the changes in spatial coverage. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus, Denmark, Finland, Luxembourg and Malta, are not included due to the geographical coverage of the Urban Audit.

Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus, Greece, Luxembourg and Malta, are not included due to the geographical coverage of the Urban Audit.

All stations in EU Member States, with at least 75 % data coverage for at least eight years were included in the analysis. Concentrations per station type are given in μg/m3. In the top two diagrams a geographical bias exists towards central Europe where there is a higher density of stations.
In the percentage frequency distribution graphs, closed bars denote stations showing a statistically significant trend, open bars denote stations with a non-significant trend. Statistically significant trends (level of significance 0.1) are calculated by applying the Mann-Kendall test. The applied method is described in de Leeuw, 2012.

The annual mean limit value is 40 µg NO2/m3. Over the years 2001-2010 the total population, for which exposure estimates are made, increased from 93 to 124 million people due to an increasing number of monitoring stations reporting air quality data under the Exchange of Information Decision. Year-to-year variations in exposure classes are partly caused by the changes in spatial coverage. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus, Greece and Malta, are not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.

The target value is 120 µg O3/m3 as daily maximum of 8 hour mean, not to be exceeded more than 25 days per calendar year, averaged over three years and to be achieved where possible by 2010. Over the years 2001-2010 the total population for which exposure estimates are made, increased from 88 to 118 million people due to an increasing number of monitoring stations reporting under the Exchange of Information Decision. Year-to-year variations in exposure classes are partly caused by the changes in spatial coverage. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus, Greece and Malta is not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.

In the air quality directive (2008/EC/50), the EU has set a target value and a long term objective value for ozone (O3) for the protection of vegetation. Target value: the AOT40 may not exceed 18000 (µg/m3) per hour in the period from 1 May to 31 July averaged over five years. Long term objective value: the AOT40 may not exceed 6000 (µg/m3) per hour in the period from 1 May to 31 July within a calendar year.
AOT40 (expressed in (μg/m3) ∙ hours) means the sum of the difference between hourly concentrations greater than
80 μg/m3 (= 40 parts per billion) and 80 μg/m3 over a given period using only the one-hour values measured
between 8.00 and 20.00 Central European Time (CET) each day. More information is provided in Annex VII of directive (2008/EC/50).